The mechanical properties of Q355B steel change with product thickness, a critical engineering consideration known as the thickness effect or size effect. In general, the guaranteed minimum yield strength, tensile strength, and impact toughness all decrease as plate thickness increases. This behavior is explicitly accounted for in the standard GB/T 1591-2018, and designers must use the thickness-specific values rather than a single handbook value.
What Is the Thickness Effect in Q355B?
The thickness effect describes the reduction of guaranteed mechanical properties with increasing section thickness. It arises because manufacturing conditions differ between thin and thick plates: thinner plates cool faster after the final rolling pass, producing a finer, stronger ferrite-pearlite microstructure, while the core of thick plates cools more slowly, allowing grains to grow coarser and weaker. Thick slabs also undergo less mechanical reduction at the core during rolling, and larger volumes of material increase the probability of inclusions, micro-voids, and chemical segregation. These metallurgical factors are the reason the standard defines properties in thickness-dependent tables.
Strength Reductions with Thickness
According to Table 2 of GB/T 1591-2018, the minimum yield strength of Q355B decreases stepwise with thickness: at least 355 MPa for thickness up to 16 mm, 345 MPa for 16-40 mm, 335 MPa for 40-63 mm, 325 MPa for 63-80 mm, and 315 MPa for 80-100 mm, with further small reductions for thicker material above 100 mm. The tensile strength is 470-630 MPa for thicknesses up to 100 mm, with slightly reduced ranges for the thickest products. As an example, a 20 mm thick plate has a minimum yield of 345 MPa, while a 100 mm plate has only 315 MPa, a reduction of about 11%. Using the 355 MPa value for a 50 mm thick member is non-conservative and unsafe.
Impact Toughness and Ductility
The guaranteed Charpy V-notch impact energy of Q355B is 34 J at +20 degrees Celsius, tested on standard 10x10 mm specimens. For thicker plates, the core cools slower and develops a coarser structure with lower toughness than the fine-grained surface, so specimens taken from the quarter-thickness or core position, as required for heavy plates under some specifications, show lower values. For critical applications such as seismic or offshore structures, thick Q355B plates may need upgrading to Q355C, Q355D, or Q355E, which guarantee impact energy at 0, -20, and -40 degrees Celsius respectively. Minimum elongation also decreases modestly with thickness, for example from about 21% in the thinner range to about 19% for the thickest plates.
Through-Thickness (Z-Direction) Properties
Thicker plates are more prone to segregation of impurities such as sulfur and phosphorus at the mid-thickness during solidification, which increases the risk of lamellar tearing at highly restrained welded connections. When the design requires through-thickness strength, standard Q355B may be insufficient, and Z-direction quality steel with guaranteed reduction of area in the thickness direction must be specified, such as Z15, Z25, or Z35 according to GB/T 5313. For heavy welded nodes, designers should also consider specifying Q355GJ, a high-performance steel with more stable strength across thicknesses, or adding supplementary requirements for through-thickness properties.
Design and Procurement Implications
In structural design, the design strength must be selected from the thickness-property table in GB/T 1591-2018 based on the actual member thickness. In procurement, the purchase order should specify both the grade and the thickness range, for example 40 to 63 mm, so the mill supplies steel meeting the correct strength guarantee. For welding of thick plates, preheat and controlled heat input become mandatory to prevent excessive hardening and cracking in the heat-affected zone, and buttering or transition joints may be considered for very thick welds. The thickness-specific minimum values from the standard are the governing values for design and construction; supplementary requirements for toughness and through-thickness properties are often necessary for critical thick-section applications.
Frequently Asked Questions
Why do the mechanical properties of Q355B change with thickness? Thinner plates cool faster after rolling, producing a finer and stronger microstructure, while the core of thick plates cools slower, allowing coarser grains to develop. Thick slabs also receive less rolling reduction at the core and contain larger volumes that may hold inclusions or segregation, all of which lower the guaranteed properties.
What is the yield strength of Q355B by thickness? Per GB/T 1591-2018, the minimum yield strength is 355 MPa up to 16 mm, 345 MPa for 16-40 mm, 335 MPa for 40-63 mm, 325 MPa for 63-80 mm, and 315 MPa for 80-100 mm, with further small reductions above 100 mm.
What is the tensile strength of Q355B? The tensile strength range is 470-630 MPa for thicknesses up to 100 mm, with slightly reduced ranges for the thickest products as defined in the standard tables.
What is the impact requirement for Q355B? Q355B guarantees a minimum Charpy V-notch impact energy of 34 J at +20 degrees Celsius. Grades Q355C, Q355D, and Q355E guarantee impact energy at 0, -20, and -40 degrees Celsius respectively.
When should Q355C/D/E or Q355GJ be specified? For thick plates in seismic moment frames, offshore nodes, or other critical applications, Q355C/D/E provide better low-temperature toughness, and Q355GJ provides more stable strength across thicknesses. Z-direction quality per GB/T 5313 should be added where through-thickness properties are required.
What is Z-direction quality steel? Z15, Z25, and Z35 designations per GB/T 5313 guarantee a minimum reduction of area in the thickness direction, which resists lamellar tearing at highly restrained welded connections in thick plates.